1. Field of the Invention
The invention relates to microphones, and more particularly to gain calibration for microphones.
2. Description of the Related Art
A microphone converts a sound into an electric signal. The electric signal generated by the microphone, however, has a small amplitude and requires to be amplified for further processing. A conventional microphone module therefore comprises a microphone and an amplification circuit for amplifying the electric signal generated by the microphone.
A conventional amplification circuit is a junction field effect transistor (JFET). Referring to FIG. 1, a block diagram of a conventional microphone module 100 is shown. The conventional microphone module 100 comprises a microphone 102, a JFET 104, and a load resistor 106. The microphone 102 is modeled as a voltage source 112 coupled between a ground and an output capacitor 114. The voltage source 112 has an amplitude VM proportional to a received sound pressure PM. A sensitivity of the microphone 102 is then determined as
      S    M    =                    ∂                  V          M                            ∂                  P          M                      .  When the microphone 102 is an electret condenser microphone (ECM), the sensitivity of the ECM is roughly −40 dBV/Pa.
The JFET 104 is biased as a common source configuration and is coupled between an output node 110 and a ground. The electric voltage output by the microphone 102 is applied to a gate of the JFET 104. The load resistor 106 is coupled between a voltage source and the output node 110. The JFET 104 can be modeled as an input capacitor 122, two diodes 124 and 126, and a PMOS 128. Therefore, an output voltage generated by the JFET 104 at the output node is according to the following algorithm:
            V      O        =                  S        M            ·                        C          O                                      C            I                    +                      C            O                              ·              G        m            ·              R        L              ,wherein VO is the output voltage, SM is a sensitivity of the microphone 102, CO is capacitance of the output capacitor 114, CI is capacitance of the input capacitor 122, Gm is a transconductance of the NMOS transistor 128, and RL is resistance of the load resistor 106.
The output voltage of the microphone module 100 at the output node 110 is therefore attenuated with increase of the capacitance of the output capacitor 114. For example, when the capacitance CO of the output capacitor 114 is 5 pF and the capacitance CI of the input capacitor 122 is 1 pF, the output voltage at the output node 110 is attenuated by 1.58 dB. When the microphone 102 is a micro-electronic-mechanical-system (MEMS) microphone, the output capacitor 114 has smaller capacitance of about 1 pF, and the output signal at the output node 110 is further attenuated; thus, degrading performance of the microphone module 100. Thus, an amplification circuit with an adjustable gain for amplifying an output signal of a microphone is required to avoid attenuation due to parasitic capacitance of the microphone.